A number of applications exists wherein it is desirable to provide monitoring capabilities for monitoring the states of a plurality of devices, such as, for example, the states of relays, switches and the like.
For example, some very severe restrictions exist in the avionics field in which it is desired to monitor a plurality of discretes, a discrete being defined as "a separate, complete and distinct signal". Such signals are typically binary, on or off, 28 V-based signals, typically: open/ground, 28 V/open or 28 V/ground and with a very, low band width, typically of the order of DC to 200 Hz.
Although the translation of such signals to TTL-levels, which are compatible with digital avionics may seem simple, lightning and high intensity radiated fields (HIRF) and RTCA DO-160C power complicate the ability to translate such signals to TTL-levels.
Present day, conventional systems utilize tailored circuits for an interface, each interface being typically comprised of R-C input filters, divider networks, diode isolation and a comparator. Multi-channel interface to a processor requires still additional logic and latches. It is further desirable to provide a standardized, addressable and reliable interface. The resulting designs generally lack built-in test capabilities consume considerable space, typically up to one square inch per channel and offer no chip-level redundancy.